HILO, Hawai'i — A U.S. Coast Guard crew fired more than 30 rounds from an M-16 rifle into a bloated whale carcass floating outside Hilo Harbor yesterday in an unsuccessful effort to sink it, but finally gave up and returned to port with the dead mammal still drifting about two miles offshore.

Lt. John Titchen said the Coast Guard will continue to monitor the 25-foot carcass to ensure it does not pose a hazard for mariners, and may try other disposal strategy if the whale drifts closer to shore.

A homeowner who was looking out to sea with binoculars spotted the carcass and reported it early yesterday, said David Schofield, Marine Mammal Response Network coordinator for NOAA's Pacific Islands Regional Office.

Titchen said the 18-foot rubber hulled Coast Guard vessel Kis 1 was summoned about 8:30 a.m. to take a team to the site to collect a tissue sample and check the carcass.

He said the decomposing whale had attracted about five tiger sharks estimated to be 12 to 17 feet long, and drew the attention of about half-dozen small boats in the area.

"It was determined it was definitely a risk to navigation, and might be a risk to the boating community," prompting Coast Guard officials to consider either towing the whale out to sea or shooting it to sink it, Titchen said.

The crew decided not to try to tow the carcass for fear it would break apart, and instead cleared the area of boats before attempting to sink it with gunfire about 1 p.m., he said.

That effort "does not appear to have worked very well right now, but in time it may," he said. He said the shots may have punctured air pockets that will cause the dead whale to sink later.

He said observers do not believe the whale is a humpback, but couldn't be certain. "It's bloated to the point it's not recognizable," he said.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.